Power-line communication (PLC) techniques enable systems, subsystems and components to communicate by exchanging information, typically through the use of radio frequency energy, over power lines whose primary purpose is the transmission of power. The information is transmitted using a power-line carrier which is typically a high-frequency signal that is superimposed on the normal waveform used for the transmission of power.
Power-line communications are commonly used for monitoring and supervisory control purposes such as remote metering, remote control of unattended power stations, automatic load control, and demand response systems. These types of command-and-control functions are increasingly important in smart-grid applications. Power-line communications are also used to transmit information that is unrelated to the underlying power system. For example, power-line communications may be used to carry voice and data traffic over existing power lines in rural areas where dedicated telephone, cable or optical transmission media may be prohibitively expensive to install.
Coupling circuits are crucial components for power-line communications because they enable the carrier signal to be superimposed on the normal power signal. A power-line coupling circuit must provide a path for information to flow between the power lines and a power-line communication circuit, while protecting the sensitive electronics in the communication circuit from the high voltage and current levels on the power lines.
FIG. 1 illustrates a typical prior-art system in which power sub-systems S1, S2, . . . , Sk, . . . , SN share an AC or DC power-line for exchanging power between the sub-systems and/or a power source/sink. Power may flow out of any of the sub-systems that generate power or convert power from other power sources, while power may flow into any of the sub-systems that consume power. Power flow may also be bi-directional into and out of any sub-systems that have the capability of both supplying and consuming power.
Each of the sub-systems S1, S2, . . . , Sk, . . . , SN includes a power-line communication module 10 to establish one or more communication channels over the power-lines and enable communications between any of the sub-systems and other components in a variety of ways such as one-to-one, one-to-many, many-to-many and many-to-one strategies. In addition, one or more network gateways 12 may be included to concentrate data and/or interface to other network functionality such as Internet access, data-servers, remote network management, etc.
In a typical system as shown in FIG. 1, the sub-systems S1, S2, . . . , Sk, . . . , SN are connected in parallel, and therefore the line communication modules 10 include power-line coupling interfaces that are designed for parallel connections. The power line current and voltage capabilities can vary based on the application. For example, in a smaller distributed control application, the voltage and current magnitudes could be a few volts to a few amperes, respectively. In other applications, these voltages can be as high as a few hundreds volts and a few hundred amperes, respectively. However, the circuitry in the on-board communication module 10 within each of the sub-systems S1, S2, . . . , Sk, . . . , SN operates at low-voltages. Thus, the circuitry must be isolated from the detrimental voltage and current levels present on the power lines, while simultaneously providing an effective path for coupling communication signals to the power lines.
FIG. 2 illustrates a prior art implementation of the on-board communication module 10 within each of the systems shown in FIG. 1. The power-line is coupled to receive and transmit sections of a communication transceiver using line interface circuit 16. The transceiver is interfaced to the sub-system through a digital data interface 18. For the transmit chain, the digital data is first modulated using special hardware, a digital signal processor (DSP), I/O multiplexer or any other suitable hardware 20. The modulated digital data is then converted to analog form using a digital-to-analog converter 22. The signal so obtained is processed with a filter 24, a buffer 26, a driver 28 and driven onto the power-line through the line interface circuits 16 which may include line-coupling circuitry such as isolation barriers as described below. For the receive section, the input signal received from the receive line coupling circuit is processed through a low-noise amplifier (LNA) 30, a filter 32 and an automatic gain-control AGC amplifier 34 before converting to digital form using an analog-to-digital converter 36. The digital data is then processed by the dedicated hardware, DSP, etc. The digital hardware 20 can interact with a variety of digital circuits for sending and receiving data to and from other parts of the system.
Some typical implementation details of the line interface circuit 16 of FIG. 2 are shown in FIG. 3. An isolation barrier 38 protects the transmit and receive signal paths from the high voltages and/or currents on the power lines. A receive line filter 40 and a transmit line filter 42 provide appropriate impedance match while simultaneously achieving correct in-band and out-band spectral characteristics. The circuit of FIG. 3 is typically arranged to provide a broadband match between the impedance seen looking into the PLC circuit and the impedance seen looking into the power line. For example, to accommodate commonly used PLC frequencies, the PLC circuit may be designed to provide flat frequency response between 50 KHz and 100 KHz with the response rolling off at rapidly below 50 KHz and above 100 KHz.
An example implementation of an isolation barrier 38 is shown in FIG. 4. A high-voltage capacitor C is arranged in series with the primary side of a transformer T1 to connect with the power-line between terminals W1 and W2. Bi-directional diode-based 44 clamps may be used to protect the circuits from surges. The secondary side of the transformer may be center tapped for creating balanced differential signals at terminals C1 and C2 relative to a ground connection at the center tap for interfacing to the receive and transmit line filters, respectively.
Some example implementation details of the receive and transmit line filters 40 and 42 are shown in FIG. 5. These circuits provide appropriate impedance match across three ports of the circuits including the interface to the receive section LNA 30, the transmit section driver 28, and the isolation barrier 38. Passive RLC receive circuits 46 are appropriately tuned to operate in specific communication bands and frequencies with appropriate bandwidths on the communication channel. The filters are collectively optimized such that the communication signals across I1 and I2 are as high as possible, but small enough to be safely connected to low-voltage levels utilized by the PLC integrated circuit (IC) chip. Similar consideration is given to the RC combinations in the transmit filter 42 for transmitting maximum power from the on-chip high-power low-impedance drivers of the transmit section. Additionally, the resistors in the transmit filter 42 may be used for current sensing. Additional protection circuitry as shown in FIG. 6 can be placed at nodes I1 and I2.
FIG. 7 illustrates the details of an entire prior art system implementation. Each of the main sub-systems S1, S2, . . . , SN has an associated PLC system including a line coupling circuit 46, receive line connect circuit 48, transmit line connect circuit 50 and a low-voltage IC communication device 52. All of the sub-systems are connected in parallel on an AC or DC power line bus. A gateway 12 is interfaced to the power-lines through interface circuit 54 which is essentially a duplicate of the PLC systems associated with each of the sub-systems.